Power control loops, such as provided in wireless communication systems, are typically provided to control and maintain output power at a desired level, and to control transient responses when ramping up or down.
In a TDMA signalling format, a framed structure comprises a number of time slots (or channels). A wireless communications system using TDMA, for example a GSM (global system for mobile telecommunications) system, transmits and receives information over each assigned slot or channel. Each channel of a frame is assigned to a different user with transmission (uplink) information on one frequency band and reception (downlink) information on a separate frequency band communicated over each channel.
Each channel is specified to ramp-up to a required power level for burst transmissions and ramp-down to a required power level in a predetermined amount of time, called a burst. Thus a burst has a ramp-up phase or cycle, an operational phase or cycle, and a ramp-down phase or cycle. GSM specifications also require that the power at the start and end of a burst must be at a specified minimum level and that the transition from the minimum power level to the final required power level must be completed in a specified time window. The time frame for ramping up and down is specified in order to reduce the generation of transient side bands and interference on adjacent channels.
The operation of a typical power control loop is divided into an open loop mode and a closed loop mode. The power control loop runs in open loop mode until the output of the power amplifier has reached a predetermined level, referred to as the switching point. At the switching point an error signal is produced using the difference of a supplied reference signal and the output of a detector which samples the signal at the output of the control loop. An integrator is used to produce a control voltage from the error signal, which is used to control a variable attenuator as a means to control power levels when ramping and during transmission bursts.
FIG. 1(a) depicts a graph of the power output of a power amplifier controlled by a conventional power control loop. During ramp-up, section A, the power output rises to the appropriate level. During transmission or operation, section B, the power output is held substantially steady by the power control loop. During ramp-down, section C, the power output falls.
A disadvantage of conventional power control loops is illustrated in FIG. 1(b). FIG. 1(b) depicts a graph of the power output of a power amplifier transmitting a non-constant envelope modulated burst, such as an AM burst. As in FIG. 1(a), during key up, section A, the power output rises to the appropriate level. However, during transmission, section B, the power output fluctuates according to the amplitude of the signal being transmitted. During key down, section C, the power output falls. A conventional power controller would tend to level out the variations in power due to the AM burst. Such a situation is unacceptable for advanced wireless transmissions using AM. A known solution allowing a conventional power control loop to be used with amplitude modulation transmissions is to add a sample and hold circuit to the power control loop. Power levels are sampled during ramping and the level is held steady during an AM burst. However, FIG. 1(b) illustrates a disadvantage of this approach when applied to advanced modulation schemes, known as gain tilt. Gain tilt can occur during a transmission burst. FIG. 1(b) depicts the gain tilt effect, showing the expected gain level at the end of the burst and the actual gain level due to gain tilt at the end of a burst. The sample and hold circuitry of a conventional power controller works on an assumption that gain does not change during a burst and does not attempt to control gain tilt during the burst.
Because of the problems that arise in the power control loop due to non-constant envelop modulation, in prior art techniques it has generally been possible only to operate in open-loop mode during the operational cycle, or to use only a very low control loop-bandwidth during the operational cycle. The latter solution leads to two particular problem. Firstly, the power control loop is unable to correct for gain variations over the burst, Secondly, because of the slow averaging, certain data patterns can cause transients at the end of the burst when restoring loop-bandwidth just prior to ramp-down.
International patent application publication number WO 01/03292 discloses one known technique directed at improving power control in systems utilising non-constant envelope modulation. The power control operation is based on the use of a sample and hold circuit. During ramp-up, a power level detected at the output of the amplifier is compared to a reference signal to give an error signal used for attenuation of the output signal. During the burst phase, the power level detected at the output of the amplifier is compared to the modulated RF signal to be transmitted to give the error signal used for attenuation of the output signal. Thus, in effect, during the burst the control loop operates in an open-loop mode, with respect to the reference signal, using a scaled envelope signal to generate the loop error signal.
One potential problem with the technique disclosed in WO 01/03292 is that there may be a small offset associated with the sample-and-hold circuit. Thus, there may be a small difference in the control signal applied for the attenuation of the output signal during switching. This may be translated into a glitch in the output signal when switching between the ramp-up period and the operational period.
It is an aim of the present invention to provide an improved power control loop which is suitable for use in non-constant envelope modulation environment but which retains all the advantages of a standard power control loop.